applied
sciences
Article
The Chemical Behaviors of Nitrogen Dioxide
Absorption in Sulfite Solution
Ye Sun, Xiaowei Hong, Tianle Zhu *, Xiaoyan Guo and Deyuan Xie
School of Space and Environment, Beihang University, Beijing 100191, China; [email protected] (Y.S.);
[email protected] (X.H.); [email protected] (X.G.); [email protected] (D.X.)
* Correspondence: [email protected]; Tel.: +86-10-8231-4215
Academic Editor: Agustín Bueno López
Received: 14 March 2017; Accepted: 6 April 2017; Published: 12 April 2017
Abstract: The simultaneous removal of nitrogen oxides (NOx ) and sulfur dioxide (SO2 ) by absorption
is considered to be one of the most promising technologies for flue gas treatment, and sulfite is
the main component of the absorption solution. To understand the chemical behaviors of the NO2
absorption in sulfite solution, the absorption time dependences of concentrations of nitrogen and
sulfur compositions in both gas phase and liquid phases were investigated by flue gas analyzer,
Ion chromatography (IC), Gas chromatography (GC), and Fourier transform infrared spectrometry
(FTIR) methods using a bubbling reactor. The mass equilibrium of the N and S compositions were
also studied. The results indicate that sulfite concentration plays a vital role in NO2 absorption.
The main absorption products are NO3 − and NO2 − , and NO2 − can be converted into NO3 − in the
presence of oxygen. Besides, about 4% to 9% by-products of S compositions are formed, and 4% to
11% by-products of N compositions such as NO, N2 , N2 O5 , N2 O, and HNO3 in the gas phase were
detected in the emissions from the bubbling reactor. On the basis of N and S compositions, a possible
pathway of NO2 absorption in sulfite solution was proposed.
Keywords: absorption; nitrogen oxides; sulfite solution; chemical behaviors; product analysis
1. Introduction
As major poisonous air pollutants, nitrogen oxides (NOx ) and sulfur dioxide (SO2 ) from the fossil
fuel combustion are key contributors to acid deposition, photochemical smog and hazy weather,
and bring out a great harm to the environment and human health [1]. NOx and SO2 removal
technologies, therefore, have been the research focus of air pollution control in recent decades. It is well
known that the commonly practiced technologies of removing NOx , such as low NOx burners, selective
non-catalytic reduction (SNCR), and selective catalytic reduction (SCR), have been widely developed
and used. SO2 emissions have also been effectively controlled by wet flue gas desulfurization (WFGD),
circulated fluid bed flue gas desulfurization (CFB-FGD), and so on. Although the combination of the
commonly practiced de-NOx and de-SO2 processes can meet the limitations of SO2 and NOx emissions
stipulated by environmental legislation, traditional NOx and SO2 removal technologies in a stepwise
manner have some drawbacks, such as the requirement for large construction site, high capital
investment, and high operation costs. Thus, the emerging technologies for the simultaneous removal
of NOx and SO2 are to be extensively developed [2,3].
The integrated oxidation-absorption process using chemical reagents for simultaneous removal
of NOx and SO2 is considered as a cost-effective and environmental friendly technology for flue gas
treatment and has gained increasing interest [4]. However, nitric oxide (NO), accounting for more than
95% of NOx in typical flue gas, is almost insoluble and low active. Thus, NO has to be oxidized into
soluble nitrogen dioxide (NO2 ) at the first step. Recently, a few studies related to the NO oxidation
have been reported [5–12], including gas phase catalytic oxidation, ozone oxidation, corona discharge
Appl. Sci. 2017, 7, 377; doi:10.3390/app7040377
www.mdpi.com/journal/applsci
Appl. Sci. 2017, 7, 377 Appl. Sci. 2017, 7, 377
2 of 10 2 of 10
oxidation have been reported [5–12], including gas phase catalytic oxidation, ozone oxidation, corona discharge oxidation, and liquid oxidation. After oxidation, the soluble NO2 can be SO
further oxidation,
and liquid
oxidation.
After
oxidation,
the soluble
NO2 can be
further
removed
with
2 in
removed with SO
2 in the subsequent absorption process [13]. Till now, the NO2 absorption process the
subsequent absorption
process [13]. Till now, the NO2 absorption process using chemical reagents
using chemical widely studied to process.
develop Many
the industrial scale process. Many have
widely
beenreagents have studied to develop
thebeen industrial
scale
absorbents
such
as Ca(OH)
2,
absorbents such as Ca(OH)
2, CaCO3, Na2SO3, (NH4)2SO3, and CaSO3 have been tested and the CaCO3 , Na2 SO3 , (NH4 )2 SO3 , and CaSO3 have been tested and the reaction mechanism have been
reaction mechanism have been probed [1,14–22]. Among them, sulfite solutions have been considered probed
[1,14–22]. Among them, sulfite solutions have been considered as a cost-effective absorbent
as a cost‐effective absorbent because of their high removal efficiency, their desulphurization byproduct because of their high removal efficiency, their desulphurization byproduct characteristics, and their
characteristics, and their capacity of simultaneous mercury chlorine [23–25]. capacity
of simultaneous
removal
of mercury
and chlorineremoval [23–25]. of However,
NOand 2 and SO2 absorption
However, NO2 and SO
2 absorption into sulfite absorbents involves complex chemical behaviors. into
sulfite absorbents
involves
complex chemical behaviors. Consequently, and the chemical behavior
Consequently, and the chemical behavior of absorption has not been widely generalized up to now. of absorption has not been widely generalized up to now. Furthermore, there are few reports on the
Furthermore, are few from
reports on the analysis of the gas products from the NO2 and SO2 analysis
of the there gas products
the NO
2 and SO2 absorption in absorbents. Thus it is absolutely
absorption in absorbents. Thus it is absolutely necessary to further understand the NO
chemical necessary to further understand the chemical behaviors
of the absorption
process
to promote
2 and
behaviors of the absorption process to promote NO
2 and SO2 removal by adsorption. SO2 removal by adsorption.
In this
this study,
study, Na
NaSO
2SO3 solution was selected as sulfite absorbent. The time‐dependences of In
2
3 solution was selected as sulfite absorbent. The time-dependences of
concentrations of main nitrogen sulfur (N and S) compositions in gas
both gas phase and phase
liquid concentrations of main nitrogen andand sulfur
(N and
S) compositions
in both
phase
and liquid
phase were investigated by flue gas analyzer, Ion chromatography (IC), Gas chromatography (GC) were investigated by flue gas analyzer, Ion chromatography (IC), Gas chromatography (GC) and
and Fourier transform spectrometry (FTIR). mass equilibrium the N and S Fourier
transform
infraredinfrared spectrometry
(FTIR). The
mass The equilibrium
of the N andof S compositions
compositions of NO
2 absorption into absorbent through mass balance calculation were also carried of NO2 absorption into absorbent through mass balance calculation were also carried out. Moreover,
2 removal was proposed. aout. Moreover, a possible reaction pathway of NO
possible reaction pathway of NO removal was proposed.
2
2.2. Experimental Section Experimental Section
2.1.
Experimental Setup
2.1. Experimental Setup AA scheme of the experimental setup is shown in Figure 1. It consists of a reaction gas feeding scheme of the experimental setup is shown in Figure 1. It consists of a reaction gas feeding
unit,
an
absorption reactor, and a set of gas analytical instruments. A stirred bubbling device with
unit, an absorption reactor, and a set of gas analytical instruments. A stirred bubbling device with an an
effective volume of 5 L (150 mm inner diameter and 300 mm in height) was used as the absorption
effective volume of 5 L (150 mm inner diameter and 300 mm in height) was used as the absorption reactor.
A mechanical agitator provided continuous stirring at a speed of 105 rpm. The temperature of
reactor. A mechanical agitator provided continuous stirring at a speed of 105 rpm. The temperature the
reactor
was maintained with the help of a thermostatic water bath. A condenser was used to cool
of the reactor was maintained with the help of a thermostatic water bath. A condenser was used to and
dehydrate the outlet flue gas and reduce the influence of H2 O on2products
analysis.
cool and dehydrate the outlet flue gas and reduce the influence of H
O on products analysis. Vent
Condenser
MFC
Temperature controller
Agitator
Flue gas analyzer
pH meter
Mixing
chamber
NO2 O2
SO2
Ar
Overflow bottle
FTIR
IC
Water bath
GC
FigureFigure 1. Schematic diagram of the experimental setup. MFC: Mass flow controller; IC: Ion 1. Schematic diagram of the experimental setup. MFC: Mass flow controller; IC: Ion chromatography;
chromatography; FTIR: Fourier transform infrared spectrometry; GC: Gas chromatography. FTIR: Fourier transform infrared spectrometry; GC: Gas chromatography.
2.2. Experimental Methods 2.2.
Experimental Methods
All experiments
experiments were
were performed
performed atat atmospheric
atmospheric pressure
pressure and
and the
the temperature
temperature ofof 325
325 ±± 55 K.
K. All
The simulated flue gas, which was supposed to be outlet flue gas after NO oxidation, was simulated The simulated flue gas, which was supposed to be outlet flue gas after NO oxidation, was simulated by
by mixing proper amount NO2, O2, SO2, and Ar from compressed gas cylinders. The basic mixing
proper
amount
of NOof 2 , O2 , SO2 , and Ar from compressed gas cylinders. The basic simulated
simulated flue gas compositions were 260 ppm NO2, 300 ppm SO2, 6% O2 and Ar. The gas flow rate Appl. Sci. 2017, 7, 377
3 of 10
flue gas compositions were 260 ppm NO2 , 300 ppm SO2 , 6% O2 and Ar. The gas flow rate was adjusted
by mass flow controller (MFC), and the four gas streams with a total flow rate of 4 L min−1 were
completely mixed in a mixing chamber before being fed into the absorption reactor. The volume of
Na2 SO3 solution (0.02 M) stored in the stirred bubbling device was 2 L. pH of absorption solution
was maintained at 6–7 by adding 1 M HCl or 1 M NaOH solution. The liquid samples at different
times for IC analysis were attained from the sampling port. Qualitative analysis of liquid phase
(NH4 + , NO3 − , NO2 − , and SO4 2− ) was carried out by ion chromatography (IC, Metrohm 792, Herisau,
Switzerland). The concentrations of NO2 , NO, SO2 , and O2 in the tail gas were monitored by a flue
gas analyzer (Testo350-pro, Schwarzwald, Germany). Sulfite ions (SO3 2− ) in the liquid phase were
measured by an iodometric titration method. N2 formed in the absorption reaction was monitored
using gas chromatography (GC, Agilent 7890, Santa Clara, CA, USA) with a Thermal conductivity
detector (TCD, Agilent, Santa Clara, CA, USA), and 5 A molecular sieve chromatographic column
(Agilent, Santa Clara, CA, USA), and the temperature of column oven and detector were 40 and
180 ◦ C, respectively. Other N compositions of outlet gas were analyzed by Fourier transform infrared
spectrometry (FTIR, Nicolet 6700, Waltham, MA, USA) equipped with a high-sensitivity mercury
cadmium telluride detectors (MCT, Nicolet, Waltham, MA, USA) cooled by liquid nitrogen. The outlet
gas stream passed through a 2.5 L gas cell, which had optical path length of 10 m, accordingly averaged
spectra after 32 scans at 4 cm−1 resolution were collected in the 3000–650 cm−1 ranges.
In the current system, inlet NO2 , SO2 concentrations, and initial sulfite concentration were
previously set, without other sources of nitrogen and sulfur compositions. The molar quantity of N
and S compositions in a closed loop system from absorption reaction could be calculated by graphical
integration during a fixed period, and the theoretical values in this loop-locked system were expressed
as follows:
Z t
Q(C0 − Ct ) × 10−6
TN t =
dt
(1)
Vm
0
TSt =
Z t
Q(C0 − Ct ) × 10−6
0
Vm
dt + 0.04
(2)
where TNt and TSt are the total molar quantity of N and S compositions absorbed into reaction system
during t absorption time (mol). C0 and Ct are inlet and outlet concentrations of NO2 or SO2 (ppm),
respectively. Q is the total flow rate of the flue gas (L min−1 ). V m is the molar volume of NO2 or SO2
in 325 K (L mol−1 ), and t is the absorption time (min). Meanwhile, 0.04 in Equation (2) is the original
molar quantity of SO3 2 − in stirred bubbling device (mol).
3. Results and Discussion
3.1. Quantitative Analysis of N and S Compositions in Gaseous and Liquid Phases
In order to analyze the distribution of N and S compositions in closed loop reaction system,
the dependence of outlet NO2 and SO2 concentrations with the absorption time were investigated,
and the results are shown in Figure 2. Due to the high sulfite concentration, both NO2 and SO2 were
not detected in the outlet flue gas during the initial absorption period of about 200 min, which suggests
that nearly all of NO2 and SO2 are absorbed. The outlet NO2 and SO2 concentrations rapidly increase
from 210 min to 230 min and are almost unchanged after 230 min. It is clear that the absorption
equilibrium is established after a certain absorption time. These results also show that that the reaction
between NO2 and sulfite may be mainly controlled by diffusion in the initial reaction period, which
is due to the sulfite concentration makes reaction rate is much faster than the diffusion rate [22,26].
Figure 2 also shows that NO, as one of the absorption products, has similar time dependence to NO2
and SO2 concentrations. It was found that hardly any NO was detected during initial absorption
period of about 200 min. NO concentration gradually increases from about 210 min to 230 min and
almost keeps unchanged after 230 min. This can be explained as follows: in the initial absorption
period, NO2 can oxidize sulfite solutions directly (R1). However, with the depletion of sulfite solutions,
Appl. Sci. 2017, 7, 377 4 of 10 absorption period, NO
2 can oxidize sulfite solutions directly (R1). However, with the depletion of Appl.
Sci. 2017, 7, 377
4 of 10
sulfite solutions, NO2 absorption in H2O (R2) occurs, and HNO2 may be formed. Generally, HNO2 is unstable in its liquid phase and can decompose into NO as R3 [22,27]. NO2 absorption in H2 O (R2) occurs, and HNO2 may be formed. Generally, HNO2 is unstable in its

2
2
liquid phase and can 2decompose
(R1) NO 2  SOinto
NO
H 2as
O R3
 [22,27].
2 NO 2  SO 4  2H  3
NOx and SO2 outlet concentration(ppm)
2NO2 + SO3 2− + H2 O → 2NO2 − + SO4 2− + 2H+
2 NO 2  H 2O  HNO 2  HNO3 2NO2 + H2 O → HNO2 + HNO3
(R1)
(R2) (R2)
3HNO
NO
 H+2O
3HNO
2NO
H2OHNO
+ HNO
2 2 2→
3 3
(R3) (R3)
300
100
250
NO2
80
200
60
150
40
NO
SO2
20
100
0
50
200
210
220
230
240
250
15
0
0
30
60
200
250
300
350
Reaction time(min)
Figure 2. The dependence of outlet NO
Figure
2. The dependence of outlet NO22, SO
, SO22, and NO concentrations with the absorption time. , and NO concentrations with the absorption time.
NO3−−, NO2−, −SO32−, 2and SO42−, which are main reactants or products of liquid phase, are NO3 , NO2 , SO3 − , and SO4 2− , which are main reactants or products of liquid phase,
synchronously analyzed by IC at the absorption times of 30 min, 60 min, 210 min, 220 min, 230 min are synchronously analyzed by IC at the absorption times of 30 min, 60 min, 210 min, 220 min,
and 300 min to better understand the chemical behaviors of NO2 and SO2 absorption in sulfite 230 min and 300 min to better understand the chemical behaviors of NO2 and SO2 absorption in
solution. As shown in Figure 3, NO3−, NO2−−, and SO
42− concentrations in absorption solution increase sulfite solution. As shown in Figure 3, NO3 , NO2 − , and SO4 2− concentrations in absorption solution
as the absorption proceeds. Compared with NO2−, more NO
3− was formed at the same absorption increase as the absorption proceeds. Compared with NO2 − , more NO3 − was formed at the same
−
time, which can be ascribed to the oxidation of NO2 and the decomposition of unstable HNO
2 [22,28]. absorption time, which can be ascribed to the oxidation of NO2 − and the decomposition of unstable
After the absorption time of about 300 min, NO2− concentration is almost unchanged although NO
3− −
HNO2 [22,28]. After the absorption time of about 300 min, NO2 concentration is almost unchanged
ions still increase, which is attributed to the fact that nitrite ions are further oxidized into nitrate ions. although NO3 − ions still increase, which is attributed to the fact that nitrite ions are further oxidized
Figure 3 also shows that the sulfite concentration gradually decreases as the absorption into nitrate ions.
proceeds and are almost unavailable after 220 min, which results in a sharp decrease of NO2 Figure 3 also shows that the sulfite concentration gradually decreases as the absorption proceeds
absorption performance. It indicates that sulfite concentration is closely related with the and are almost unavailable after 220 min, which results in a sharp decrease of NO2 absorption
performance of the removal of NO2, which agrees well with the findings shown in Figure 2. performance. It indicates that sulfite concentration is closely related with the performance of the
The sulfite is consumed because of its reaction with NO2 or O2. For NO2 absorption in sulfite removal of NO2 , which agrees well with the findings shown in Figure 2. The sulfite is consumed
solution, the sulfate is formed through the reaction between NO2 or O2 and sulfite (R1) [29]. because of its reaction with NO2 or O2 . For NO2 absorption in sulfite solution, the sulfate is formed
through the reaction between NO2 or O2 and sulfite (R1) [29].
Appl.
Sci. 2017, 7, 377 Appl. Sci. 2017, 7, 377 55 of 10 of 10
Appl. Sci. 2017, 7, 377 5 of 10 0.06
2-
SO
2- 3
0.016
SO3
0.04
0.04
2-
SO
2- 4
SO4
NO3-
0.012
0.012
NO3-
NO
NO
- 22
The amount of Sulfur species (mol)
0.016
0.02
0.02
0.00
0.00
0.008
0.008
-0.02
-0.02
0.004
0.004
-0.04
-0.04
0.000
0.000
30
30
60
210
60
220
210
230
220
Reaction time(min)
230
The amount of Sulfur species (mol)
0.06
0.020
The amount of Nitrogen species (mol)
The amount of Nitrogen species (mol)
0.020
-0.06
300
-0.06
300
Reaction time(min)
Figure 3. The dependence of SO
42−, NO3−−
, and NO2− contents with the reaction time. 2− 32−, SO
2−
−
Figure 3. The dependence of SO3 2−, SO4 2− , NO3− , and NO−2 contents with the reaction time.
Figure 3. The dependence of SO3 , SO4 , NO3 , and NO2 contents with the reaction time. 3.2. Equilibrium Analysis of Nand S Compositions 3.2.
Equilibrium Analysis of Nand S Compositions
3.2. Equilibrium Analysis of Nand S Compositions Figures 4 and 5 present the N and S equilibrium of NO2 and SO2 absorption in sulfite solution, The amount of Nitrogen species (mol)
Figures
4 andIt 5can present
thefrom N and
S equilibrium
of NO
and
SO22NO
absorption
in sulfite
solution,
respectively. be seen Figure 4 that the sum of NO
3− and 2− ions in the absorption 22 and SO
Figures 4 and 5 present the N and S equilibrium of NO
absorption in sulfite solution, − and NO − ions in the absorption
respectively.
It
can
be
seen
from
Figure
4
that
the
sum
of
NO
solution is lower than the calculated TN
t for almost all the reaction times, which suggests that other 3 3− and NO22− ions in the absorption respectively. It can be seen from Figure 4 that the sum of NO
N compositions of about 4% to 11%, are formed in the absorption process. Some researchers have solution
is
lower
than
the
calculated
TN
for
almost
all
the
reaction
times, which suggests that other
t
solution is lower than the calculated TNt for almost all the reaction times, which suggests that other confirmed the existence of to
other N are
compositions. Siddiqi et al. [30] process.
studied the effect of nitrogen have
N
compositions
of
about
4%
11%,
formed
in
the
absorption
Some
researchers
N compositions of about 4% to 11%, are formed in the absorption process. Some researchers have dioxide on the absorption of sulfur dioxide in wet flue gas cleaning processes and pointed out that confirmed
the
Siddiqi et et al. al. [30] [30] studied studied the the effect effect of
confirmed the existence
existence of
of other
other N
N compositions.
compositions. Siddiqi of nitrogen
nitrogen nitrogen–sulfur compounds could be formed and further decomposed into gaseous products in the dioxide
on
the
absorption
of
sulfur
dioxide
in
wet
flue
gas
cleaning
processes
and
pointed
out that
dioxide on the absorption of sulfur dioxide in wet flue gas cleaning processes and pointed out that co‐existence condition of NO2 and SO2 absorption process. Yan, et al. [31] also reported that the nitrogen–sulfur
compounds
could
be
formed
and
further
decomposed
into
gaseous
products
in the
nitrogen–sulfur compounds could be formed and further decomposed into gaseous products in the reaction of sulfite with NO could produce N2. However, few quantitative researches about gas phase co-existence
condition of of NO
and SO
SO22 absorption absorption process. process. Yan, Yan, et et al. al. [31] [31] also also reported reported that
products have been previously reported. In addition, Figure 4 also shows that the amount of other N co‐existence condition NO22 and that the
the reaction
of
sulfite
with
NO
could
produce
N
.
However,
few
quantitative
researches
about
gas
phase
2
compositions is almost kept constant before 210 min, and then decreases as the absorption proceeds, reaction of sulfite with NO could produce N
2. However, few quantitative researches about gas phase products
have
beenthat previously
reported.
In addition,
Figure
4 also of shows
the amount of
other N
which means sulfite plays a dominant role in the formation other that
N compositions. These products have been previously reported. In addition, Figure 4 also shows that the amount of other N 2−
results are consistent with the dependence of NO
2 removal (Figure 2) and SO
3 amount (Figure 3) compositions
is almost kept constant before 210 min,
and then decreases as the
absorption proceeds,
compositions is almost kept constant before 210 min, and then decreases as the absorption proceeds, with the reaction time. which
means
that
sulfite
plays
a
dominant
role
in
the
formation
of
other
N
compositions.
These results
which means that sulfite plays a dominant role in the formation of other N compositions. These 2 − amount (Figure 3) with the
are
consistent
with
the
dependence
of
NO
removal
(Figure
2)
and
SO
2− amount (Figure 3) 2
3
TN
results are consistent with the dependence of NO
2
removal (Figure 2) and SO
3
0.012
t
Other nitrogen species
4.4%
reaction
time.
with the reaction time. NO 2-
0.010
NO 3-
TN t
10.2%
Other nitrogen species
NO 2-
The amount of Nitrogen species (mol)
0.012
0.008
0.006
0.010
NO 310.2%
0.004
0.008
6.7%
4.4%
7.8%
6.7%
11%
0.002
0.006
7.8%
11.1%
0.000
0.004
30
60
210
220
Reaction time(min)
11%
0.002
230
300
Figure 4. The equilibrium analysis of N compositions for NO2 absorption in sulfite solution. 11.1%
0.000
30
60
210
220
230
30032− and SO42−, as shown in Similarly, the calculated TSt is also more than the sum of detected SO
Reaction time(min)
2−
2− Figure 5. Other S compositions besides SO3 and SO4 account for about 4% to 9% in the reaction system. Besides the above‐mentioned nitrogen‐sulfur compounds, Chen et al. [32] and Zhuang et al. Figure 4. The equilibrium analysis of N compositions for NO
Figure
4. The equilibrium analysis of N compositions for NO22 absorption in sulfite solution. absorption in sulfite solution.
[32,33] thought that the sulfite radicals could be formed by the reaction between NO
2 and sulfite, the sulfite radicals could undergo either recombination or reaction with oxygen. By comparing Figure 4 2−
Similarly, the calculated TSt is also more than the sum of detected SO3 2−
and SO42−, as shown in Similarly,
the calculated TSt is also more
than
the
sum
of
detected
SO
and SO4 2− , as shown
with Figure 5, it can be seen that the amount of other S compositions is much lower than that of other 3
2−
2− Figure 5. Other S compositions besides SO3 and SO4 account for about 4% to 9% in the reaction in Figure 5. Other S compositions besides SO3 2− and SO4 2− account for about 4% to 9% in the
system. Besides the above‐mentioned nitrogen‐sulfur compounds, Chen et al. [32] and Zhuang et al. reaction system. Besides the above-mentioned nitrogen-sulfur compounds, Chen et al. [32] and
[32,33] thought that the sulfite radicals could be formed by the reaction between NO2 and sulfite, the Zhuang et al. [32,33] thought that the sulfite radicals could be formed by the reaction between NO2 and
sulfite radicals could undergo either recombination or reaction with oxygen. By comparing Figure 4 with Figure 5, it can be seen that the amount of other S compositions is much lower than that of other Appl. Sci. 2017, 7, 377
6 of 10
Appl. Sci. 2017, 7, 377 6 of 10 sulfite,
the sulfite radicals
could undergo either recombination or reaction with oxygen. By comparing
Figure
4 with Figure 5,
than that
Appl. Sci. 2017, 7, 377 it can be seen that the amount of other S compositions is much lower6 of 10 compositions, which which
suggests that not S compositions participate in in
the formation ofN other
N compositions,
suggests
thatall notother all other
S compositions
participate
the
formationof N compositions, which suggests that not all other S compositions participate in the formation of other N compositions. of other N compositions.
other N compositions. 0.07
TSt
0.06
TSt
Other
Sulfur species
2- Sulfur species
Other
SO32-
0.05
SO42-
The amount of Sulfur species (mol)
The amount of Sulfur species (mol)
0.07
0.06
SO3
3.7%
2-
SO4
0.05
0.04
0.04
8.0%
7.1%
3.7%
6.0%
8.0%
7.1%
6.0%
210
220
230
8.9%
8.9%
8.3%
8.3%
0.03
0.03
0.02
0.02
0.01
0.01
0.00
30
0.00
60
30
60 Reaction
210time(min)
220
230
300
300
Reaction time(min)
Figure 5. The equilibrium analysis of S compositions for SO2 absorption in sulfite solution. Figure 5. The equilibrium analysis of S compositions for SO2 absorption in sulfite solution.
Figure 5. The equilibrium analysis of S compositions for SO2 absorption in sulfite solution. 3.3. Qualitative Analysis of N Compositions 3.3.
Qualitative Analysis of N Compositions
3.3. Qualitative Analysis of N Compositions Besides NO2, NO, NO2−, and NO3−, the nitrous products such as N2O, N2O3, N2O5, and N2, can − , and NO − , the nitrous products such as N O, N O , N O , and N ,
Besides
NO2 ,2, NO, NO
NO, 2NO
form during the NO
absorption into sulfite solution [10,30]. IC, GC, and FTIR were used to detect 22−, and NO
2
23, N
3 2O25, and N
5
2
Besides NO
33−, the nitrous products such as N2O, N
2O
2, can these products in this study. Figures 6 and 7 present the [10,30].
GC and IC,
FTIR spectra of the were
gaseous can
form
during
the
NO
absorption
into
sulfite
solution
GC,
and FTIR
used to
2
form during the NO2 absorption into sulfite solution [10,30]. IC, GC, and FTIR were used to detect products, respectively. detect
products
in this
study.
Figures
6 and
7 presentthe theGC GCand and FTIR FTIR spectra
these these
products in this study. Figures 6 and 7 present spectra of
of the
the gaseous
gaseous It can be seen from Figure 6 that the gaseous N2 is formed mainly in the initial absorption products,
respectively.
products, respectively. period, declines as the absorption proceeds, and finally disappears after 230 min. N2 formation can It
that
the
gaseous
NN
2 2is
It can
can bebe seen
seen from
from Figure
Figure 6 6 that the gaseous is formed
formed mainly
mainly inin the
the initial
initial absorption
absorption be explained by the following reactions [31], period,
declines
as
the
absorption
proceeds,
and
finally
disappears
after
230
min.
N
formation
can be
2
period, declines as the absorption proceeds, and finally disappears after 230 min. N2 formation can 2
2
explained
by
the
following
reactions
[31],
(R4) 2 NO  2SO 3  2SO 4  N 2 be explained by the following reactions [31], N O  SO
2
2  SO
2
2 N (R5) 2
3
4
2− 2SO
2−2 N 2 NO
 2+SO
2NO
2SO
2SO
N
3 3 →
4 4 +
2 2
(R4) (R4)
2 The sulfite plays an important role in N
2 2 formation according to Reactions (4) and (5). Thus N
2− SO 2 2
−N (R5) N 2N
Othe SO
O fact + SO
→ SO
+ 2N2
3that 4 concentration 2
3
4
concentration decrease is due to sulfite in reaction system gradually (R5)
declines and finally 2 concentration cannot contribute to the equilibrium The
sulfite
plays
andisappears. important Although role in N2Nformation
according to Reactions (4) and (5). Thus N2 2 2 formation according to Reactions (4) and (5). Thus N
The sulfite plays an important role in N
analysis of nitrogen, the observed declining tendency of N2 peaks with the absorption time indicates concentration
decrease
is due
to the
concentration
in reaction
system gradually
declines
concentration decrease is due to fact
the that
fact sulfite
that sulfite concentration in reaction system gradually that N2 is one of other nitrogen compositions. and
finallyand disappears.
Although N
cannot contribute
the equilibrium
of
declines finally disappears. Although N2 concentration cannot to
contribute to the analysis
equilibrium 2 concentration
nitrogen,
the
observed
declining
tendency
of
N
peaks
with
the
absorption
time
indicates
that
N
analysis of nitrogen, the observed declining tendency of N
2 peaks with the absorption time indicates 2
2 is
N
one
of other
nitrogen compositions.
that N
2 is one of other nitrogen compositions. 30 min
O +Ar
2
60 min
210 min
220 min
230 min
300 min
Indensity (V)
N2
Indensity (V)
O2+Ar
N2
0
2
4
Time(min)
6
8
Indensity (V)
Indensity (V)
2
5.0
N2
30 min
60 min
210 min
220 min
230 min
300 min
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Time(min)
Figure 6. GC spectra of gaseous N‐products during NO2 absorption. 0
2
4
6
8
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Based on the FTIR analysis, the effect of absorption time on the gaseous N‐product distribution Time(min)
Time(min)
−1, 2287 cm−1, can be seen, as shown in Figure 7. Six distinct absorbance peaks are observed at 2930 cm
1720 cm−1, 1600 cm−1, 1326 cm−1, and 887 cm−1, and the bands are assigned to NO2, N2O, N2O5, NO2, Figure 6. GC spectra of gaseous N‐products during NO
2 absorption. Figure
6. GC spectra of gaseous N-products during NO2 absorption.
Based on the FTIR analysis, the effect of absorption time on the gaseous N‐product distribution can be seen, as shown in Figure 7. Six distinct absorbance peaks are observed at 2930 cm−1, 2287 cm−1, 1720 cm−1, 1600 cm−1, 1326 cm−1, and 887 cm−1, and the bands are assigned to NO2, N2O, N2O5, NO2, Appl. Sci. 2017, 7, 377
7 of 10
Based on the FTIR analysis, the effect of absorption time on the gaseous N-product distribution
can be seen, as shown in Figure 7. Six distinct absorbance peaks are observed at 2930 cm−1 , 2287 cm−1 ,
1720 cm−1 , 1600 cm−1 , 1326 cm−1 , and 887 cm−1 , and the bands are assigned to NO2 , N2 O, N2 O5 ,
Appl. Sci. 2017, 7, 377 7 of 10 NO2 , HNO3 , and HNO3 , respectively [34]. In the initial absorption period, the intensities of bands at
−1 , 1720 cm−1 , 1326 cm−1 , and 887 cm−1 almost keep unchanged, which shows that N O, N O ,
2287 cm
2
2 5
HNO
3, and HNO3, respectively [34]. In the initial absorption period, the intensities of bands at 2287 and
HNO
as
gaseous
products
are
continuously
generated
from
the
absorption
reactions.
Then
the
−1
−1
−1
−1
3
cm , 1720 cm , 1326 cm , and 887 cm almost keep unchanged, which shows that N2O, N2O5, and amounts
of
N
O,
N
O
,
and
HNO
rapidly
decrease
after
the
absorption
time
of
about
210
min
and
2
2 5
3
HNO3 as gaseous products are continuously generated from the absorption reactions. Then the finally
disappear
after
about
230
min,
which
indicates
that the absorption equilibrium is established,
amounts of N2O, N2O5, and HNO3 rapidly decrease after the absorption time of about 210 min and and
no
sufficient
sulfite
is
used
to
react
with
NO
to
produce
gaseous N2 O, N2 O5 , and HNO3 . These
2
finally disappear after about 230 min, which indicates that the absorption equilibrium is established, results
are
consistent
with
the
equilibrium
analysis
of
sulfur
compositions
5). In fact, not all
and no sufficient sulfite is used to react with NO2 to produce gaseous N2(see
O, NFigure
2O5, and HNO3. These sulfite would be converted to sulfate ions, nitrogen–sulfur compounds, and sulfite radical may exist in
results are consistent with the equilibrium analysis of sulfur compositions (see Figure 5). In fact, not reaction system. In addition, the occurrence of N2 O5 is due to the reaction between NO2 and O2 in gas
all sulfite would be converted to sulfate ions, nitrogen–sulfur compounds, and sulfite radical may phase,
or the decomposition of nitrogen–sulfur compounds.
Furthermore, the gaseous HNO3 is
not
exist in reaction system. In addition, the occurrence of N
2O5 is due to the reaction between NO
2 and only
attributed to the fact that HNO3 in of absorption
liquid enters
gas phase,
but also that
gas
phase
O
2 in gas phase, or the decomposition nitrogen–sulfur compounds. Furthermore, the gaseous reaction
happens,
which
can
be
described
by
the
following
Reaction
(6):
HNO3 is not only attributed to the fact that HNO3 in absorption liquid enters gas phase, but also that gas phase reaction happens, which can be described by the following Reaction (6): 3NO2 (g) + H2 O(g) → NO(g) + 2HNO3 (g)
(R6)
3NO 2 (g)  H 2O(g )  NO(g )  2HNO3 (g ) (R6) Figure 7 also shows that the peak at 1600 cm−1 , assigned to NO2 , gradually increases and the
−1, assigned to NO2, gradually increases and the peakFigure 7 also shows that the peak at 1600 cm
at 1900 cm−1 , assigned to NO, gradually decreases
after the absorption time of about 210 min.
−1
peak at 1900 cm
, assigned to NO, gradually decreases after the absorption time of about 210 min. The results are also
in accordance with those of gas phase composition analysis by flue gas analyzer
The results are also in accordance with those of gas phase composition analysis by flue gas analyzer (Testo350-pro, Germany), as shown in Figure 2.
(Testo350‐pro, Germany), as shown in Figure 2. NO2
-1
Intensive (a.u.)
(1600cm )
300 min
230 min
220 min
210 min
60 min
30 min
HNO3
HNO3
-1
-1
(887cm ) (1326cm )
800
1200
800
1200
NO
-1
(1900cm )
N2O5
N2O
-1
(1720cm )
-1
(2287cm )
1600
2000
2400
1600
2000
2400
2800
NO2
-1
(2930cm )
2800
3200
-1
Wavenumber(cm )
Figure 7. FTIR spectra of the gaseous N‐products during NO
Figure 7. FTIR spectra of the gaseous N-products during NO22 absorption. absorption.
Figure 8 shows the distribution of nitrogen products for NO2 absorption into sulfite solution, Figure 8 shows the distribution of nitrogen products for NO2 absorption into sulfite solution,
based on mass balance for a typical inlet gas with 260 ppm NO2, as the sole N source. Because NO2 is based on mass balance for a typical inlet gas with 260 ppm NO2 , as the sole N source. Because NO2
hardly detected in the outlet flue gas of the reactor before 60 min, NO2 absorbed into the liquid phase is hardly detected in the outlet flue gas of the reactor before 60 min, NO2 absorbed into the liquid
is 2.56 mmol during initial absorption time of 60 min. NO3− −and NO2− −concentrations in the phase is 2.56 mmol during initial absorption time of 60 min. NO3 and NO2 concentrations in the
absorption solution, based on IC results, are 1.511 and 0.766 mmol, respectively. Because both NH3 absorption solution, based on IC results, are 1.511 and 0.766 mmol, respectively. Because both NH3
and NH4++ have not been found by FTIR and IC, the other N compositions is calculated as 0.283 mmol and NH4 have not been found by FTIR and IC, the other N compositions is calculated as 0.283 mmol
according to mass balance, account for 11% of the total N compositions, and the calculated values of according to mass balance, account for 11% of the total N compositions, and the calculated values of
other N compositions content are in good agreement with the experimental values. other N compositions content are in good agreement with the experimental values.
Appl. Sci. 2017, 7, 377
8 of 10
Appl. Sci. 2017, 7, 377 8 of 10 Appl. Sci. 2017, 7, 377 NO3-
＋
NO3-
＋
(IC, 1.511 mmol)
Inlet NO2
(2.56 mmol)
Inlet NO2
(2.56 mmol)
NO2-
8 of 10 (IC , 0.766 mmol)
NO2-
(IC , 0.766 mmol)
(IC, NH
1.511
3 mmol)
＋ N2 ＋ N2O ＋ N2O5＋ HNO3
(FTIR, Null)
(GC)
(FTIR)
NH3
NH
+
4
(FTIR, Null)
(IC, Null)
＋
N2 ＋ N2O ＋ N2O5＋ HNO3
(GC)
(0.283 mmol) (FTIR)
NH4+
(0.283 mmol)
(IC, Null)
Figure 8. The nitrogen balance of NO
Figure
8. The nitrogen balance of NO22 absorption in sulfite solution. absorption in sulfite solution.
3.4. NO2 Absorption Pathway 3.4. NO2 Absorption Pathway
Figure 8. The nitrogen balance of NO2 absorption in sulfite solution. According to the above‐mentioned analyses, a possible NO2 absorption pathway can be According
to the above-mentioned
analyses, a possible NO2 absorption pathway can be proposed,
proposed, as depicted in Figure 9. 3.4. NO
2 Absorption Pathway as depicted
in
Figure
9.
The main reactants in the gas phase include NO2, SO2, and O2. The sole N source in the inlet flue According to the the
above‐mentioned analyses, a SO
possible 2 absorption pathway can be The
gas phase
include
NO2 ,sulfite ONO
N source
in NO
the2 inlet
2 , and
2 . The
gas main
is NOreactants
2. When in
NO2 laden gas contacts with solution, the sole
transfer of the into flue
proposed, as depicted in Figure 9. gas is absorption NO2 . Whensolution NO2 laden
gasmeanwhile, contacts with
sulfite take solution,
of phase, the NOand absorption
occurs, reactions place the
in transfer
the liquid the main 2 into
The main reactants in the gas phase include NO
2, SO2, and O2. The sole N source in the inlet flue products of meanwhile,
NO2 absorption in flue take
gas are NOin
3− and 2−, especially NOthe
3− in main
absorption solution solution
occurs,
reactions
place
the NO
liquid
phase, and
products
of NO2
gas is NO2. When NO2 laden gas contacts with sulfite solution, the transfer of the NO2 into −. HNO3 is considered −
−
−
dramatically increases due to the absorption of NO
2
and the oxidation of NO
2
absorption
in flue
gas are
NO3 meanwhile, and NO2reactions , especially
inthe absorption
solution
dramatically
3 in absorption solution occurs, take NO
place liquid phase, and the main − . HNO is considered
to involve gas and liquid states existing in reaction system, and the gaseous HNO
3 in outlet flue gas increases
due
to
the
absorption
of
NO
and
the
oxidation
of
NO
to involve
−
−
−
2
2
3
products of NO2 absorption in flue gas are NO3 and NO2 , especially NO3 in absorption solution is from both HNO3 escape in absorption solution and the reaction between NO
2 and H2O in the gas gas and
liquid states existing in reaction system, and
the gaseous HNO3 in
outlet
flue gas is from
dramatically increases due to the absorption of NO
2 and the oxidation of NO
2−. HNO
3 is considered phase. HNO2 may be produced by NO2− combining with H+, and HNO2 is unstable in liquid phase to involve gas and liquid states existing in reaction system, and the gaseous HNO
3 in outlet flue gas both HNO
O in the gas phase.
3 escape in absorption solution and the reaction between NO2 and H
and easily decomposes, which leads to HNO3 formation and NO release. As 2 NO increasing, − combining with H+ , and HNO is unstable in2 and H
3 escape in absorption solution and the reaction between NO
2phase
O in the gas HNOis from both HNO
may
be
produced
by
NO
liquid
and easily
2a homogeneous reaction of NO and NO
2
2
2 results in an increase of the N
2O3 to a smaller extent, the NO phase. HNO
2 may be produced by NO2− combining with H+, and HNO2 is unstable in liquid phase decomposes,
which
leads
formation and NO release. As NO increasing, a homogeneous
or N2O3 can react with to
SOHNO
32− to 3form hydroxylamine‐disulfonate (HADS), which is a precursor of and ofeasily decomposes, which leads to HNO
formation NO release. As NO increasing, reaction
NO and
NO2 results
in an
increase
of3the
N2 O32 in the gas phase is in favor of the sulfite toand a smaller
extent,
the
NO
or N2 O3 can
numerous N‐S compounds. In addition, the existence of SO
a homogeneous reaction of NO and NO
2 results in an increase of the N2O3 to a smaller extent, the NO 2
−
react with
SO3 toformation, form hydroxylamine-disulfonate
is a HADS. precursor
numerous N-S
and bisulfate and HNO2 can also react (HADS),
with HSOwhich
3− to form The ofintermediate or N2O3 can react with SO32− to form hydroxylamine‐disulfonate (HADS), which is a precursor of products of N‐S compounds finally decompose to N
2
O
5
, N
2
O, and N
2
in gas phase. compounds. In addition, the existence of SO2 in the gas phase is in favor of the sulfite and bisulfate
numerous N‐S compounds. In addition, the existence of SO2 in the gas phase is in favor of the sulfite formation,
and HNO
HSO − to form HADS. The intermediate products of N-S
2 can also
and bisulfate formation, and react
HNOwith
2 can also 3react with HSO3− to form HADS. The intermediate compounds
finally decompose to N2 O5 , N2 O, and N22Oin
gas
phase.2 in gas phase. products of N‐S compounds finally decompose to N
5, N
2O, and N
Figure 9. A possible NO2 absorption pathway. Figure 9. A possible NO2 absorption pathway. Figure
9. A possible NO2 absorption pathway.
Appl. Sci. 2017, 7, 377
9 of 10
4. Conclusions
A series of experiments were performed to study the chemical behaviors for the NO2 and SO2
absorption in sulfite solution, and a possible NO2 absorption pathway was proposed. The main
findings can be summarized as follows: The performance of NO2 absorption is closely related with
sulfite concentration. The main N and S compounds generated in the NO2 absorption process are
NO3 − , NO2 − , and SO4 2− . Moreover the NO3 − , NO2 − , and SO4 2− concentrations change with
absorption times. Besides, about 4% to 9% by-products of S compositions are formed, and 4% to
11% by-products of N compositions, such as NO, N2 , N2 O5 , N2 O, and HNO3 in gas phase, were
detected in the emissions. Meanwhile, N2 , N2 O5 , and N2 O could be ascribed to the decomposition of
sulfur—nitrogen compounds as intermediate products exited from the redox reaction between sulfite
solutions and NO2 .
Acknowledgments: The authors gratefully acknowledge the financial support of the National Natural Science
Foundation of China (No. 21377009).
Author Contributions: Tianle Zhu conceived and designed the experiments; Ye Sun performed the experiments;
Ye Sun, Xiaowei Hong, Xiaoyan Guo and Deyuan Xie analyzed the data; Ye Sun and Tianle Zhu wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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